The invention relates to a method and a device for inspecting a fuel element in a nuclear reactor. Such a fuel element includes a bundle of fuel rods. At one end of the fuel rods is a head part, and at the other end a foot part. Situated between the head and foot parts are spacers disposed one above another at axial spacings. Fuel elements in boiling water reactors are usually further surrounded by boxes.
As a rule, these fuel elements have a square cross section, that is to say the outer surfaces of the fuel element, which are formed by the outer surfaces of the head and foot parts and of the spacers or the fuel element box, are situated opposite one another in pairs. In the ideal case, two outer surfaces of a spacer are in each case parallel to one another and to the corresponding outer surfaces of the fuel element foot or head.
Wear phenomena and damage to the fuel elements can occur during operation of the reactor. Thus, for example, the cladding tube wall of the fuel rods can corrode and/or water can penetrate into individual fuel rods.
The intensive neutron emission to which the fuel element is exposed leads to a radiation-induced growth of the fuel rods and, possibly, also of the fuel element box. Inhomogeneities in the distribution of the thermal energy and the neutron flux render the growth in length dependent on location, and this can lead to bending, bowing and twisting of the fuel element. Due to irradiation and corrosion, the webs from which the spacers are produced are also subjected to growth which depends, moreover, on the rolling direction during the rolling-out of the sheet metal used.
Since the fuel elements in the reactor core are seated at a mutual spacing of only a few millimeters, such changes falsify the physical states for which the reactor operation is configured. Moreover, there are problems in removing and inserting fuel elements when the spacers have become wider or are deformed in a barrel-shaped fashion.
Usually, spent fuel elements are extracted from the reactor core in time intervals of approximately one year. The remaining fuel elements are relocated, and samples of them are checked for damage. This investigation must be carried out under water, since the irradiated fuel element is highly radioactive and must be cooled because of the development of heat during the decay of fission products. To date, the underwater inspection has mainly been undertaken using video cameras with the aid of which it is possible to detect external damage to the fuel elements such as, for example, spacers with broken corners. U.S. Pat. No. 4,605,531 discloses devices which scan the fuel elements with the aid of ultrasonic probes. For such investigations, the fuel element is brought into a defined position relative to the probes used. It is possible thereby to find damage to fuel rod cladding tube walls into which water has penetrated.
It is accordingly an object of the invention to provide a method and a corresponding device for inspecting a fuel element which overcome the above-mentioned disadvantages of the heretofore-known methods and devices of this general type and which allow to detect changes in a fuel element in a simple way.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for inspecting an irradiated fuel element in a nuclear power plant, the method includes the steps of:
measuring, with a measuring device, a spacer of a fuel element for providing measurements of the spacer;
measuring, with the measuring device, a calibration rod for providing measurements of the calibration rod, the calibration rod having known dimensions; and
calibrating the measurements of the spacer by using the measurements of the calibration rod.
In accordance with another mode of the invention, a measured value for a spacing between the outer surfaces and a further measured value for a spacing between the subareas is formed for respective two points situated opposite one another on two outer surfaces of the spacer pointing in opposite directions and respectively situated opposite one another on two subareas of the calibration rod pointing in the opposite directions; and the measured value for the spacing between the outer surfaces is converted into a calibrated measured value by using the known dimensions of the calibration rod and the further measured value for the spacing between the subareas.
In accordance with yet another mode of the invention, further subareas of the calibration rod are measured, the further subareas pointing in the opposite directions and being provided offset relative to the subareas of the calibration rod.
In accordance with a further mode of the invention, further subareas of a further calibration rod are measured, the further subareas pointing in the opposite directions and being provided offset relative to the subareas of the calibration rod.
With the objects of the invention in view there is also provided, a method for inspecting an irradiated fuel element, the method includes the steps of:
providing, in a nuclear power plant, a fuel element having a bundle of fuel rods, end pieces respectively configured as a foot piece and a head piece at respective ends of the bundle, a spacer penetrated by the fuel rods between the end pieces, and further structural elements;
positioning the fuel element with one of the end pieces or the spacer against a frame, the frame defining a z-axis of a Cartesian reference system;
holding, on the frame, a calibration rod having known dimensions in an x-direction of the Cartesian reference system;
measuring a respective relative position of two outer surfaces of the spacer and of corresponding subareas of the calibration rod, the two outer surfaces of the spacer extending along a y-direction of the Cartesian reference system; and
forming, with the aid of the known dimensions of the calibration rod, at least one calibrated maximum value for a spacing between the two outer surfaces of the spacer from measured values obtained during the measuring step.
In accordance with another mode of the invention, a relative position of the fuel element is varied in the Cartesian reference system for measuring further outer surfaces of the spacer by using probes and the calibration rod.
In accordance with yet another mode of the invention, the probes measure further outer surfaces of the spacer and corresponding further subareas of the calibration rod, the further outer surfaces and the further subareas extending along the x-direction.
In accordance with a further mode of the invention, the probes measure subareas of a further calibration rod, the subareas of the further calibration rod extending along the x-direction.
In accordance with another mode of the invention, further subareas of the calibration rod are measured, the further subareas being offset relative to the subareas of the calibration rod.
In accordance with another mode of the invention, given subareas of a further calibration rod are measured; and further given subareas of the further calibration rod are scanned, the further given subareas being offset relative to the given subareas of the further calibration rod.
With the objects of the invention in view there is also provided, a device for inspecting an irradiated fuel element in a nuclear reactor, including:
a positioning device for positioning a fuel element having a spacer with two outer surfaces pointing in opposite directions;
a measuring device operatively connected to the positioning device, the positioning device fixing a relative position of the spacer relative to the measuring device, the measuring device having a calibration rod with known dimensions and two subareas respectively pointing in the opposite directions, the measuring device being directed toward the two outer surfaces of the spacer and the two subareas of the calibration rod for forming measured values defining relative positions of the outer surfaces and the subareas; and
a computer connected to the measuring device and storing a reference value for the known dimensions of the calibration rod, the computer or computing device being configured such that the measured values and the reference value for the known dimensions of the calibration rod are used to determine and display at least one maximum spacing between the outer surfaces, the at least one maximum spacing being calibrated with respect to the stored reference value.
In accordance with another feature of the invention, the measuring device includes two probes disposed opposite from one another and a drive for moving the probes, the drive moves the probes synchronously along the outer surfaces of the spacer and the subareas of the calibration rod.
In accordance with yet another feature of the invention, the measuring device includes a plurality of mutually oppositely disposed probes, the probes simultaneously generating measured values for a plurality of points on the outer surfaces of the spacer and the subareas of the calibration rod.
In accordance with a further feature of the invention, the measuring device includes further probes and a further calibration rod with further subareas pointing in opposite directions, the further probes are directed toward mutually oppositely disposed points on two further outer surfaces of the spacer, the two further outer surfaces respectively point in a same direction as the further subareas of the further calibration rod.
In accordance with another feature of the invention, the measuring device includes a drive for positioning the calibration rod against the spacer.
With the objects of the invention in view there is also provided, in combination with a fuel element having a bundle of fuel rods, a structural head part and a structural foot part at respective ends of the bundle, and a plurality of spacers provided between the structural head part and the structural foot part, a device for inspecting the fuel element, including:
a frame defining a z-direction of a Cartesian coordinate system;
a plane table connected to the frame, the plane table being displaceable in an x-y-plane of the Cartesian coordinate system and having two arms extending in a y-direction of the Cartesian coordinate system, the arms being configured such that one of the spacers of the fuel element can be positioned therebetween;
at least one pair of probes fitted at mutually opposite positions on the arms;
at least one calibration rod movable in the y-direction;
a computer connected to the probes, the computer storing a reference value for a length of the at least one calibration rod;
probes measuring a dimensional extension of the one of the spacers in an x-direction of the Cartesian coordinate system and the length of the at least one calibration rod; and
the computer calculating at least one maximum expansion of the one of the spacers in the x-direction and calibrating the at least one maximum expansion with respect to the reference value.
In accordance with another feature of the invention, the plane table is displaceable in the z-direction and wherein one of the structural foot part and the structural head part can be positioned with respect to the frame for positioning the fuel element with respect to the frame.
In accordance with yet another feature of the invention, the frame has a base plate displaceable in the z-direction, the plane table is exchangeably mounted on the base plate, and a drive displaces the plane table in the x-direction and the y-direction on the base plate.
The invention is based on the finding that changes which are caused by the growth in the individual components of the fuel element, and other dimensional changes, for example bowing, bending and twisting of fuel element structural parts (such as spacers and fuel element boxes), impair the functionality and are therefore to be detected during inspection and measured.
According to the invention, the irradiated fuel element bundle is inspected by measuring at least one spacer of the fuel element and a calibration rod with known dimensions jointly in a measuring device. The measurements on the calibration rod serve the purpose in this case of calibrating the measurements on the spacer.
In particular, the contour of the spacer and the calibration rod can be measured in a punctiform fashion. Two points which are situated opposite one another on two first outer surfaces, pointing in opposite directions, of the spacer, can be assigned, by the measurement, a measured value for the spacing between these outer surfaces. If, for example, an ultrasonic pulse emanating from a probe is reflected at a point on the outer surface, the spacing between the point and the probe is proportional to the propagation time of the pulse-echo. Thus, if the two opposite points are measured through the use of opposite ultrasonic probes whose spacing A is known, the spacing S between two opposite points, (that is to say, in practice, the width of the spacer at this point) is given by the formula
S=Axe2x88x92c(dt1+dt2)
c representing the (temperature-dependent) propagation rate of the ultrasound, and dt1 and dt2 respectively representing the propagation time of the respective pulse-echo, that is to say the measured value of the measuring device. This propagation rate c can be determined in the concrete case by also measuring two points which are situated opposite one another on subareas of the calibration rod and likewise point in this direction. The same formula
d0=Axe2x80x2xe2x88x92c(dt1xe2x80x2+dt2xe2x80x2)
holds for this measurement of the calibration rod, Axe2x80x2 now being the known spacing between the probes directed to the subareas, and dt1xe2x80x2 and dt2xe2x80x2 respectively being the propagation times of the echo which is generated in each case at these points on the subareas. However, the spacing do between these subareas is known, and so the relationship
c=(Axe2x80x2xe2x88x92d0)/(dt1xe2x80x2+dt2xe2x80x2) results.
This relationship therefore permits the measured values dt1 and dt2, which are obtained on the outer surfaces of the spacer, to be converted into geometrical spacings.
In the case of other position pickups, the measuring device supplies, for example, measured voltages or other variables which are generally not proportional but can be converted into geometrical variables by a characteristic function (xe2x80x9ccalibration curvexe2x80x9d). At least one assignment of a second measured value to a known geometrical variable is required for the purpose of determining this calibration curve. Such further points on the calibration curve can, however, be measured with the aid of the calibration rod (or a further calibration rod), if second (or further) subareas are also measured which point in opposite directions and are provided offset relative to the first subareas.
The method is carried out, in particular, under water, that is to say the measuring device with the calibration rod is disposed under water and can contain at least two opposite probes, in order to measure the outer surfaces and subareas in the way outlined. This operation can be observed by a video camera. The measurements are measured in a computer connected to the measuring device. The image picked up by the video camera is displayed on a display screen, for example together with suitably selected, calibrated measured values. The calibrated measured value for the maximum spacing between mutually opposite outer surfaces of the spacer is of particular interest as characteristic quantity.
The use of two probes which simultaneously scan two mutually opposite points on the outer surfaces of the spacer lying therebetween has the advantage that the mutual spacing between the points (that is to say the width of the spacer) is formed by subtracting the probe signals. This spacing is then independent of which spacings there are between the probes and the outer surfaces; systematic measuring errors can thus cancel one another out. Moreover, temporary influences which could falsify a measurement are also compensated, since the measurement is calibrated with each measuring operation.
A corresponding inspection device therefore contains a measuring device with a calibration rod of known dimensions, a positioning device and a computer. The positioning device fixes the relative position of a spacer of a fuel element with respect to the measuring device, and the measuring device is directed toward two first outer surfaces, pointing in opposite directions, of the spacer and two first subareas, pointing in these directions, of the calibration rod. The measuring device can therefore form measured values for the relative position of the outer surfaces and subareas. The computer is configured in such a way that the measured values and a stored reference value for the dimensions of the calibration rod are used to determine and display at least one maximum spacing between the outer surfaces, the at least one maximum spacing being calibrated through the use of this reference value.
The positioning device has a holder into which the fuel element can be inserted in the direction of its longitudinal axis, that is to say in the vertical direction, and is also fixed in the horizontal direction. The positioning device also advantageously contains a positioning drive with the aid of which the vertical position of the measuring device with the calibration rod can be varied. It is then possible to measure sequentially a plurality of spacers of the fuel element and, if appropriate, also the foot part and/or head part.
The measuring device advantageously also contains a second drive with the aid of which the calibration rod can be laid against the spacer.
The measuring device advantageously contains a plurality of mutually opposite probes which simultaneously form measured values for a plurality of points on the first outer surfaces and subareas. It is possible to use this device to determine with high accuracy all characteristic quantities of the spacer which depend not on the relative position of the spacer outer surfaces in the measuring device, but only on the mutual position of these outer surfaces.
A pair of mutually opposite outer surfaces of the spacer can be measured in the way outlined so far. However, a square spacer has a further pair of outer surfaces, which likewise point in opposite directions. These can be measured through the use of the same measuring device and the same calibration rod when the position of the spacer with reference to the measuring device is appropriately rotated. Therefore, it is also possible to use a measuring device which contains further probes which are directed toward these second outer surfaces and corresponding second subareas of the calibration rod (or a further calibration rod).
In a specific variant of the method, the fuel element is positioned with an end piece (for example the head or the foot) or a spacer on a frame which fixes a z-axis of a Cartesian reference system. Held on this frame is the calibration rod, whose dimensions in the x-direction of the Cartesian reference system are known. The spacer and the calibration rod are positioned in the frame in such a way that two first outer surfaces of the spacer and two first subareas of the calibration rod extend along the y-direction. Measuring these outer surfaces and subareas yields measured values from which, in the way already described, through the use of the known dimensions of the calibration rod, at least one calibrated maximum value is formed for the spacing between the two first outer surfaces of the spacer.
It is advantageous when, for the purpose of measuring, two mutually opposite probes are guided synchronously along the first outer surfaces of the spacer and the first subareas of the calibration rod, giving rise sequentially in the process to the formation at least of measuring signals which correspond to the mutual spacing between two opposite points on the outer surfaces of the spacer and the subareas of the calibration rod. The measuring signals for the known spacing between the subareas are then used in order to convert the measuring signals for the outer surfaces into calibrated measured values automatically in a computer.
However, it is also possible to use a plurality of probes for measuring which are situated opposite one another in pairs and are used simultaneously to measure a plurality of points situated opposite one another in pairs, on the first outer surfaces of the spacer and the first areas of the calibration rod (preferably also on second subareas of the same calibration rod or another calibration rod). It is possible in this way also to measure a pair of second outer surfaces of the spacer which extend along the x-direction. The measured values obtained on these second outer surfaces can be calibrated in the way described through the use of the calibration rod already used for the first outer surfaces, but it can be advantageous for the purpose of increasing the accuracy also to measure second subareas of the calibration rod (or another calibration rod) which likewise extend along the x-direction.
A corresponding specific device therefore contains a frame which fixes the z-axis of a Cartesian coordinate system, and a plane table which can be displaced in the x-, y-plane of the Cartesian coordinate system and has two arms which extend in the y-direction and between which a spacer of the fuel element can be positioned. These arms contain at least one pair of mutually opposite probes. Furthermore, at least one calibration rod is provided which can be moved in the y-direction and can be held on the frame independently of the plane table or can also be part of the plane table itself. Connected to the probes is a computer in which a reference value for the length of the calibration rod is stored.
The probes can then be used to measure the expansion of the spacer in the x-direction and the length of the calibration rod, the computer being configured such that it is possible to calculate there at least one maximum expansion of the spacer in the x-direction, the at least one maximum expansion being calibrated through the use of the reference value.
The fuel element is advantageously positioned with the foot or head on the frame, while the plane table can be displaced in the z-direction.
In particular, it is advantageous when the plane table is mounted exchangeably on a base plate which can be displaced in the z-direction, and at least the arms can be displaced in the x-direction and y-direction with reference to the base plate through the use of a drive. When the plane table is dismounted, the frame can be used with the base plate as carrier for further devices with the aid of which other measurements and/or repair work are undertaken on the fuel element.
Thus, by contrast with the ideal dimensions of the components of the fuel element, the invention can be used also to measure deviations in the dimensions which are 20 xcexcm or less. It is then possible to decide reliably whether the geometry of the fuel element allows its further usexe2x80x94if appropriate, by replacing deformed spacers, or relocating a deformed fuel element into a position in the core at which the radiation and/or mechanical load leads to a deformation of the fuel element which cancels out again the measured deformation.
In addition, the relative position of the outer surfaces with respect to the corresponding outer surfaces at the end of the bundle, that is to say the head part and/or foot part of the fuel element, can also be measured. Bowing or twisting of the fuel rod bundle can thereby be detected. For example, a bowed bundle can then be rotated by 180xc2x0 and be reinserted in the same position, after which it is possible to wait until it bows back again during further operation. Even in a boiling-water fuel element whose box is still scarcely deformed, it is, specifically, possible for a bowed or twisted fuel rod bundle to have the effect that the bundle is no longer optimally centered in the box.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for inspecting a nuclear reactor fuel element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.